Find out what we're doing about managing flood risks in Hutt City.

Hutt City is the most densely populated flood plain in New Zealand. The city has a history of flooding, and the projected effects of climate change makes flooding a complex and difficult natural hazard to manage.

To give us a clearer understanding of stormwater flood risks in Hutt City and better prepare for the future, Wellington Water (which manages water services for the Wellington Region’s councils) is modelling the likely impacts of extreme rainfall on the urban areas of Hutt City.

Wellington Water

What we're doing to manage flood risk

Council is currently planning for the forecast impacts of climate change and population growth on our water infrastructure out to the next 30 years.

  • we've set aside $62.8 million over the next 10 years for stormwater upgrades and improvements.
  • council manages flood risk in a variety of ways. The stormwater network of pipes and pumps is designed to drain normal levels of rainfall away from properties, into waterways and on to the coast.
  • there are also some retention ponds and wetlands that catch a certain amount of flood water when the system is under pressure.
  • historically, storm water networks were designed to carry away water during low to medium intensity rainfall events. When rainfall exceeds the pipe capacity, then water flows overland.
  • new buildings are required to include stormwater retention tanks to store water during high rainfall events to delay when stormwater enters rivers and streams.
  • Wellington Water’s modelling of flood hazards across Hutt City will provide crucial data for Hutt City Council for future urban planning to minimise risk, as well as for planning future stormwater and other infrastructure investment.

Identifying at-risk areas of flooding

Computer modelling identifies areas that are affected by heavy rainfall draining from the hills into urban areas, from roofs, roads, footpaths and the water's overland paths to drains, streams and rivers. We convert this information into maps.

The computer modelling:

  • is done to best practice standards,
  • takes into account the forecasted impacts of climate change out to 2120 and
  • is being peer reviewed by independent experts.

The resulting stormwater flood hazard maps depict flooding in 1 in 100 year and 1 in 10 year floods. They show:

  • where waterways are most likely to breach their banks,
  • where water moves across land during flooding and
  • areas of ponding.

The work represents the most comprehensive flood hazard mapping in the city’s history. It is expected to be completed by the end of 2021. The modelling of urban Wainuiomata is the first model to be completed.

This is now being followed by Stokes Valley, Petone and Alicetown. In the coming months, the Waiwhetū catchment, which includes most of the Te Awa Kairangi/Hutt River valley floor, will be completed.

Flood hazard maps

Wellington Water has started modelling the likely impacts of extreme rainfall on the urban areas in Hutt City, starting with Wainuiomata.

Flood hazard maps show what flooding looks like for flooding in 1-in-100-year and 1-in-10-year floods along with:

  • where waterways are most likely to breach their banks
  • where water moves across land during flooding, and
  • areas of ponding.

Storm water flood maps

How maps help with flooding

The maps will help reduce the impact of flooding by giving council, the community and government agencies a better understanding of future flood events. This means we can improve infrastructure, our urban planning and our responses during extreme rainfall events.

An important part of finalising the maps is asking local people about their experiences of flooding on their properties and in their neighbourhoods. Wellington Water and council staff will be showing draft maps to residents and seeking their observations and photos of past events at public drop-in sessions to ensure the maps are as accurate as possible.

Next steps

The first modelling to be completed is of urban Wainuiomata. To ensure the maps are as accurate as possible, Wellington Water and Hutt City Council held two public drop-in sessions.

This enabled residents to view the maps, ask questions about the project and provide their observations and photos of past flooding events. This information was used to fine tune the maps, which are now finalised.

A similar process is now underway for the rest of Hutt City. Please see venues and dates at the top of this page.

For more information

Email Wellington Water: floodmap@wellingtonwater.co.nz.